I will also use a high quality 3/4 plywood with hardwood veneer exterior.
The recommendations you got on the internet are probably sound. But who has the time, skill, resources and money to build with marble or granite? You could certainly get a counter top installer to do it but that is hardly going to fit the decor of the existing shelves you mentioned.
Like wise, 2.5 inches of MDF is simply going to be ugly if that thickness is visible anywhere.
To be honest, I’d just focus on making them sturdy and let your basic vibration reduction techniques handle the isolation tasks. Thats what I’m going to do.
I think the primary area you would want to focus on in terms of isolation is how your shelves will be suspended. My middle shelf, which the pre-amp will sit on, will be adjustable. If you will have adjustable shelves you could always add strips of sorbothane to the underside of the shelf that contacts the support pegs or brackets.
I’m not into vinyl to any serious extent so isolating a TT is not one of my issues. My amp weighs 100 pounds so that will be an issue for me. You could easily get a piece of granite from a counter top maker and have it sitting on the top shelf with sorbothane feet under it and then whatever else you use to isolate the TT under the TT resting on the granite.